Nonhuman primates of the species Erythrocebus patas (patas), Macaca mulatta (rhesus), and Macaca fascicularis (cynomolgus) are subjected to direct-acting and metabolism-dependent chemical carcinogens by transplacental or direct exposure, and in some cases subsequently to agents that promote development of specific neoplasms in rodents. Pathologic changes, including neoplasms, are evaluated by histopathology and ultrastructural methods. Mechanisms of organ and species differences in the effects of chemical carcinogens and tumor promoters in rodent and nonhuman primate species are investigated.